This invention relates generally to radio frequency identification tag devices, and more particularly, to a radio frequency identification tag device with a sensor input which modifies a tag data word bitstream read by a tag interrogator/detector.
Radio frequency identification (RFID) tag devices may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards and the like. RFID tag devices will work under more hostile environmental conditions than bar code labels since the RFID tag device may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tag devices are used in conjunction with a radio frequency tag reader (interrogator) which generates a continuous wave (CW) radio frequency (RF) carrier that activates the RFID tag device at close range. The RFID tag device is passive and has no internal power sources, rather it uses some of the power in the CW RF carrier to power internal circuits that read a stored internal digital code and cause the RFID tag device to signal its stored internal digital code to the tag reader.
The RFID tag device modifies the amplitude of the CW carrier of the interrogator by loading and unloading a resonant circuit which is tuned to the CW carrier. The RFID tag device comprises, for example, a parallel resonant circuit or antenna tuned to the frequency of the radio frequency CW carrier, an RF to direct current (DC) converter, a circuit for loading and unloading the parallel resonant circuit/antenna, logic which stores the internal digital code, logic which reads the internal digital code and causes the circuit for loading and unloading the parallel resonant circuit/antenna to operate in co-operation with the internally stored digital code.
The internal digital code of the RFID tag device, like information on a bar code, is normally fixed and cannot be changed except by command from the interrogator. There are situations where an RFID tag device is associated with a piece of equipment such as a valve, a process monitor, etc., having variable status or information. This variable status or information may be, for example, from a binary on/off switch(es); an analog sensor detecting pressure, temperature, voltage, current, speed, moisture, pH, etc.; or a digital value from a counter or output of an analog to digital converter. Generally, means other than the RFID tag device is required to read this variable status or information. For example, contact or connection with the switch, sensor or digital value; visually reading a display value, or using an active RF system which continuously consumes power.
Therefore, what is needed is a more cost effective, simple and reliable system, method and apparatus for obtaining variable status or information from a tagged item.
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing in an RFID tag device a sensor input adapted to receive variable signals from a switch(es), an analog variable or a digital variable. Typically, the RFID tag device amplitude modulates the CW RF carrier of the RF generator with its data word bitstream by loading and unloading the resonant tuned circuit or antenna of the RFID tag device in accordance with the binary values of that data word bitstream. The data word bitstream is a series of on/off pulses which represent, for example, a serial data word synchronization header, the RFID tag number, and, according to the embodiments of the invention, a sensor value(s). Parity bits or a checksum value may also be incorporated into the data word bitstream. These series of on/off pulses are detected by the tag reader/interrogator which determines amplitude variations of its CW RF signal. These amplitude variations are caused by the electro-magnetically coupled or RF antenna coupled RFID tag device loading and unloading the tuned circuit or antenna, respectively, of the tag reader/interrogator.
In accordance with an embodiment of the present invention, an RFID tag device has a digital input (on/off) for sensing a discrete change in voltage, current or resistance of a sensor connected to the digital input. The digital input sense state may determine whether a bit is set or cleared in the data word bitstream or whether the data word bitstream bit values may be inverted. Either way, the difference between the two data word bitstreams represent the change in the sensor (open or closed) which represents whatever the sensor represents, i.e., open or closed valve, circuit breaker on or tripped, and the like. A source of voltage or current for the sensor may be obtained from an external source, or from the RFID tag device itself which then supplies some of the power from the electro-magnetically coupled or RF antenna coupled CW from the interrogator/tag reader. The sensor may be an electromechanical switch, a transistor, a hall effect device, a photo-transistor and the like.
Another embodiment of the RFID tag device has a plurality of digital inputs (on/off) for sensing discrete changes in voltage, current or resistance of sensors or circuits connected to the plurality of digital inputs. The sense states of the digital inputs determine which ones of a plurality of bits are set or cleared in the data word bitstream. A source of voltage or current for the sensors or circuits connected to the digital inputs may be obtained from an external source or from the RFID tag device itself which uses some of the power from the electro-magnetically coupled or RF antenna CW from the interrogator/tag reader. The sensors or circuits connected to the digital inputs may be electromechanical switches, transistors, hall effect devices, photo-transistors, the output of an analog-to-digital converter, and the like.
Still another embodiment of the RFID tag device has an analog input for sensing an analog sensor signal represented by a variable voltage, current or resistance value. The analog input may be converted to a digital value by an analog-to-digital converter which is also part of the circuits of the RFID tag device. The analog input may also be converted to an on/off, high/low representation by a voltage comparator. A plurality of analog inputs may be used with the present invention, each analog input having a digital value. The plurality of analog inputs may be processed through an analog multiplexer into the analog-to-digital converter, and/or a voltage comparator. Voltage or current for powering the analog sensor(s) may be obtained from an external source, or from the RFID tag device which uses some of the power from the electro-magnetically coupled or RF antenna coupled CW from the interrogator/tag reader. The analog sensor(s) may be an RTD, thermocouple, piezo-electric pressure transducer and the like.
Yet another embodiment of the RFID tag device has a sensor built into the tag device itself. The sensed value may be for example: pressure, temperature, acceleration, vibration, moisture content, gas percentage, density, flow rate, light intensity, sound intensity, radiation, magnetic flux, pH, etc. The converted digital value read from the sensor input is transmitted as part of the data word bitstream. The analog output of the sensor may be converted to a digital value by an analog-to-digital converter within the RFID tag device. The analog sensor output may also be converted to an on/off, high/low representation by a voltage comparator. A plurality of analog sensor outputs may be converted within the RFID tag device into digital values. The plurality of analog sensor outputs may be processed through an analog multiplexer into an analog-to-digital converter, and/or a voltage comparator. Voltage or current for powering the analog sensor(s) may be obtained from an external source, or from the RFID tag device which then supplies some of the power from the electro-magnetically coupled or RF antenna coupled CW from the interrogator/tag reader.
Any of the aforementioned embodiments may incorporate dual-purpose external connection pads in which the pad has one function before setting a lock function in the RFID tag device, and a separate function after locking the device or writing to a particular register. For example, a dual-function pad may be used as a reset function before locking a memory array, and as a digital input after locking the memory array.
The RFID tag device may be fabricated onto a single semiconductor integrated circuit die, or it may be comprised of a plurality of semiconductor dice in a single integrated circuit package. It is also contemplated and within the scope of the invention that multi-device RFID tags incorporating a plurality of discrete electronic devices, including but not limited to, microcontrollers, memories, digital logic circuits, analog circuits, and discrete and/or monolithic transducer/sensors, may be fabricated into any of the aforementioned embodiments.